Gasket molding system for membrane electrode assemblies

ABSTRACT

The present invention is a system for molding a gasket to a membrane electrode assembly. The system comprises a cavity defined at least in part by closable mold blocks, at least one injection gate for injecting gasket material into the cavity, a mount for retaining the membrane electrode assembly adjacent to the cavity, and a mold insert independently movable relative to the closable mold blocks for applying pressure to the membrane electrode assembly retained on the mount.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. Ser. No. 11/272,887, filed Nov.14, 2005 now U.S. Pat. No. 7,771,181, now allowed, the disclosure ofwhich is incorporated by reference in its entirety herein.

FIELD OF THE INVENTION

The present invention relates to membrane electrode assemblies for usein electrochemical devices, such as fuel cells. In particular, thepresent invention relates to systems for molding gaskets to membraneelectrode assemblies.

BACKGROUND OF THE INVENTION

Fuel cells are electrochemical devices that produce usable electricityby the catalyzed combination of a fuel such as hydrogen and an oxidantsuch as oxygen. In contrast to conventional power plants, such asinternal combustion generators, fuel cells do not utilize combustion. Assuch, fuel cells produce little hazardous effluent. Fuel cells converthydrogen fuel and oxygen directly into electricity, and can be operatedat higher efficiencies compared to internal combustion generators.Because individual fuel cells do not produce much energy (e.g., betweenabout 0.7-0.9 volts), multiple fuel cells may be arranged together in astack to generate enough electricity to operate motor vehicles andsupply electricity to remote locations.

A fuel cell, such as a proton exchange membrane (PEM) fuel cell,typically contains a membrane electrode assembly (MEA) formed by acatalyst coated membrane disposed between a pair of gas diffusionlayers. The catalyst coated membrane itself typically includes anelectrolyte membrane disposed between a pair of catalyst layers. Therespective sides of the electrolyte membrane are referred to as an anodeportion and a cathode portion. In a typical PEM fuel cell, hydrogen fuelis introduced into the anode portion, where the hydrogen reacts andseparates into protons and electrons. The electrolyte membranetransports the protons to the cathode portion, while allowing a currentof electrons to flow through an external circuit to the cathode portionto provide power. Oxygen is introduced into the cathode portion andreacts with the protons and electrons to form water and heat.

MEAs are typically sealed with gaskets to prevent pressurized gases andliquids from escaping. To ensure that the pressurized gases and liquidsdo not bypass the electrolyte membranes, the gaskets are generallymolded around the peripheral edges of the MEAs. However, a common issuewith gasket molding systems is that the systems may over-compress orunder-compress the MEAs. Over-compression may cause the anode portionsand the cathode portions of the MEAs to contact through the respectiveelectrolyte membranes, resulting in electrical shorts. Alternatively,under-compression may result in gasket materials being molded inundesirable locations around the MEAs. Accordingly, there is a need fora gasket molding system that reduces the risk of over-compressing andunder-compressing MEAs during gasket molding operations.

BRIEF SUMMARY OF THE INVENTION

The present invention relates to a system for molding a gasket to anMEA. The system includes a mold cavity defined at least in part byclosable mold blocks, and at least one injection gate for injectinggasket material into the mold cavity. The system further includes amount for retaining the MEA adjacent to the mold cavity, and a moldinsert independently movable relative to the closable mold blocks forapplying pressure to the MEA retained on the mount. The system iscapable of providing suitable levels of pressure to MEAs during gasketmoldering operations, thereby reducing the risk of over-compression andunder-compression.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an isometric side view of a molding system of the presentinvention.

FIG. 2 is a side sectional view of the molding system.

FIG. 3 is an expanded view of section 3 taken in FIG. 2, showing agasket molded onto an MEA within a cavity of the molding system.

FIG. 4 is a cutaway perspective view of an alternative MEA suitable foruse with the molding system, and a gasket molded onto the alternativeMEA.

FIGS. 5 a-5 c are top view illustrations of alternative electrolytemembranes suitable for use with the molding system.

FIG. 6 is an expanded sectional view of a portion of an alternativemolding system of the present invention.

FIG. 7 is an expanded view of section 7 taken in FIG. 6, showing agasket molded onto an MEA within a cavity of the alternative moldingsystem.

FIG. 8 is a cutaway perspective view of an alternative MEA suitable foruse with the alternative molding system, and a gasket molded onto thealternative MEA.

FIGS. 9 a and 9 b are expanded sectional views of a portion a two-shotmolding system of the present invention.

While the above-identified drawing figures set forth several embodimentsof the invention, other embodiments are also contemplated, as noted inthe discussion. In all cases, this disclosure presents the invention byway of representation and not limitation. It should be understood thatnumerous other modifications and embodiments can be devised by thoseskilled in the art, which fall within the scope and spirit of theprinciples of the invention. The figures may not be drawn to scale. Likereference numbers have been used throughout the figures to denote likeparts.

DETAILED DESCRIPTION

FIG. 1 is an isometric side view of molding system 10 of the presentinvention, which is a suitable system for molding gaskets onto MEAs. Asshown, molding system 10 includes top block 12, bottom block 14, moldinsert 16, mount 18, and process control unit 20. Top block 12 andbottom block 14 are closable injection mold blocks, where top block 12is capable of moving along axis A to open and close against bottom block14 with the use of standard motion systems, such as with hydraulic,mechanical, or pneumatic systems. In an alternative arrangement, bottomblock 14 may be the molding block capable of moving along axis A and topblock 12 is stationary. Directional orientations such as “top” and“bottom” are used herein for ease of discussion, and are not intended tobe limiting.

Top block 12 includes perimeter wall 22, which extends laterally aroundand defines recessed portion 24. Top block 12 also includes opening 26(shown in phantom) through which mold insert 16 is movably retained.Recessed portion 24 is a recess within perimeter wall 22 that extendslaterally around mold insert 16. Recessed portion 24 includes patternedsurface 28, which includes a plurality of replicated patterns to formgaskets having “raised-ridge microstructured contact patterns” and“replicated structures” as disclosed in Wald et al., U.S. PatentApplication Publication No. 2003/0211378, and in the pending U.S. PatentApplication entitled, “Gasketed Subassembly For Use In Fuel Cells”, bothof which are commonly assigned.

Mold insert 16 includes contact surface 30, which is a planar surfacefor applying pressure to an MEA (not shown) retained within moldingsystem 10 during a gasket molding operation. Contact surface 30 of moldinsert 16 may include a compliant surface (e.g., a rubber surface) toincrease the uniformity of pressure applied across the MEA.Alternatively, contact surface 30 of mold insert 16 may include aperimeter lip (not shown) extending around the lateral edges of contactsurface 30. In this embodiment, the central portion of contact surface30 is recessed from the perimeter lip (e.g., about 200 micrometers orless), and the perimeter lip contacts and applies pressure around thelateral edges of an MEA retained within molding system 10 during agasket molding operation.

As discussed below, mold insert 16 may move along axis A within opening26 independently of top block 12. This allows mold insert 16 toindependently adjust the pressure applied to an MEA, thereby reducingthe risk of over-compressing and under-compressing the MEA. Mold insert16 may be moved with standard motion systems, such as with hydraulic,mechanical, or pneumatic systems. Such motion systems may communicatewith process control unit 20 via signal line 20 a. Process control unit20 is an automated system that monitors the pressure mold insert 16applies to a given MEA. The applied pressure may be monitored with aforce sensor, such as a pressure pad or a load cell (e.g., strain-gaugeload cells), which communicates with process control unit 20 via signalline 20 b. Process control unit 20 may be any suitable system forcontrolling the movement of mold insert 16 based on the sensed appliedpressure. In alternative embodiments, the motion of mold insert 16 maybe manually controlled without the use of process control unit 20.

Bottom block 14 includes perimeter wall 32, which extends laterallyaround and defines recessed portion 34. Recessed portion 34 is a recesswithin perimeter wall 32 that extends laterally around mount 18, similarto recessed portion 24. Recessed portion 34 includes patterned surface36, which includes the same replicated patterns as patterned surface 28.In alternative embodiments, patterned surface 28 and 26 may incorporatediffering patterns, or alternatively, one or both of patterned surface28 and 26 may be free of replicated patterns.

Mount 18 is an insert that is secured to bottom block 14, and is alignedwith mold insert 16. In an alternative embodiment, mount 18 may beintegrally formed with bottom block 14. Mount 18 includes surface 38 forretaining an MEA during a gasket molding operation. Surface 38 of moldinsert 16 may also include a compliant surface (e.g., a rubber surface)to increase the uniformity of pressure applied across the MEA.

As further shown in FIG. 1, top block 12 also includes injection gates40, which are openings in patterned surface 28 for injecting gasketmaterial. While shown with a pair of injection gates 40, molding system10 may include a plurality of injection gates at a variety of locations,such as in patterned surfaces 28 and 36, in perimeter walls 22 and 32(exiting into recessed portions 24 and 34, respectively), andcombinations thereof. In one embodiment, injection gates 40 may belocated at positions in the resulting gasket where manifold openingswill be created (for facilitating gas and/or liquid transport throughthe gasket).

As discussed below, molding system 10 is suitable for molding gasketsonto MEAs, where mold insert 16 may be adjusted along axis A to reducethe risk of over-compressing and under-compressing the MEAs. Thispreserves the structural integrity of the MEAs, and improves theconnections between the MEAs and the gaskets.

FIG. 2 is a side sectional view of molding system 10, showing top block12 closed against bottom block 14 with MEA 42 disposed between moldinsert 16 and mount 18. For ease of discussion, the thickness of thelayers of MEA 42 are exaggerated in FIG. 2. During a gasket moldingoperation, MEA 42 may be placed on mount 18 in an automated or manualmanner. When MEA 42 is retained on mount 18, top block 12 may be closedagainst bottom block 14 to define cavity 44. Cavity 44 extends laterallyaround MEA 42 and has a volume corresponding to recessed portions 24 and34 (shown in FIG. 1). Additionally, when top block 12 closes againstbottom block 14, contact surface 30 of mold insert 16 presses againstMEA 42, which compresses MEA 42 against surface 38 of mount 18.

Top block 12 closes against bottom block 14 with a preset amount offorce to effectively seal perimeter 22 against perimeter 32. Thisprevents gasket material from exiting molding system 10 during a gasketmolding operation. Consequently, if mold insert 16 was affixed to topblock 12 (i.e., not independently movable), the relative distancebetween contact surface 30 of mold insert 16 and surface 38 of mount 18would be constant for every injection run. This would cause the pressureapplied to an MEA retained on mount 18 (e.g., MEA 42) to vary dependingon the layer thickness of the given MEA. For example, if a given MEA hasa high layer thickness (e.g., 1,000 micrometers), the MEA may beover-compressed, potentially resulting in an electrical short.Alternatively, if the given MEA has a low layer thickness (e.g., 200micrometers), the MEA may be under-compressed, which may lead toinjected gasket material undesirably flowing between the MEA andsurfaces 30 and 38.

Mold insert 16, however, is independently movable relative to top block12 (and bottom block 14). Therefore, the pressure applied by mold insert16 to MEA 42 may be held constant by adjusting the position of moldinsert 16 along axis A. This reduces the risk of over-compressing orunder-compressing MEA 42 during a gasket molding operation. Accordingly,while top block 12 closes against bottom block 14, mold insert 16 maymove toward mount 18 and MEA 42 along with top block 12. During thistime, process control unit 20 monitors the pressure that mold insert 16applies in real-time.

When mold insert 16 contacts MEA 42, the monitored pressure applied toMEA 42 correspondingly increases. Mold insert 16 continues to compressMEA 42 against mount 18 until a preset, desired pressure is reached.Process control unit 20 then holds MEA 42 at that location relative tobottom block 14 and mount 18, regardless of the movement of top block12. For example, if MEA 42 has a high layer thickness, mold insert 16may reach the desired pressure and hold its position before top block 12reaches bottom block 14. This reduces the risk of over-compressing MEA42 while top block 12 continues to move toward bottom block 14.Alternatively, if MEA 42 has a low layer thickness, mold insert 16 maycontinue to compress MEA 42 after top block 12 is sealed against bottomblock 14, until the desired pressure is reached. This correspondinglyreduces the risk of under-compressing MEA 42.

When top block 12 and bottom block 14 are sealed together, and when moldinsert 16 has compressed MEA 42 to the desired pressure, gasket materialmay be injected into cavity 44 through injection gates 40. Examples ofgaskets materials that may be used to form gasket 46 include elastomericmaterials, such as rubbers, silicone elastomers, thermoplasticelastomers, thermoset elastomers, elastomeric adhesives,styrene-containing diblock and triblock copolymers, and combinationsthereof.

The injected gasket material substantially fills cavity 44 and conformsto the peripheral edges of MEA 42 and patterned surfaces 28 and 36. Uponsolidification, the gasket material forms a gasket (not shown), which issecured to the peripheral edges of MEA 42. Additionally, the gasket hasreplicated patterns formed by patterned surfaces 28 and 36, as discussedabove, which improve the sealing efficiency of the gasket.

FIG. 3 is an expanded view of section 3 taken in FIG. 2, which showsgasket 46 formed within cavity 44. As further shown in FIG. 3, MEA 42includes electrolyte membrane 48 disposed between gas diffusion layers50 and 52, where gas diffusion layers 50 and 52 extend beyond theperipheral edge of electrolyte membrane 48 to define gap 53. When thegasket material is injected into cavity 44, the gasket materialpenetrates into gap 53 to strengthen the connection between MEA 42 andthe resulting gasket 46.

A common issue with injectable gasket materials is that the viscosityand other flow characteristics of a given gasket material may varysignificantly between production batches. As a result, the force of theinjected gasket material may vary between injections, which may affecthow far the gasket material penetrates into gap 53. To account for this,the position of mold insert 16 may be adjusted to correspondingly adjustthe pressure applied to MEA 42. This allows the desired amount of gasketmaterial to penetrate into gap 53. For example, if the gasket materialhas a high viscosity, the pressure that mold insert 16 applies to MEA 42may be reduced to reduce the force required to penetrate into gap 53.Alternatively, if the gasket material has a low viscosity, the appliedpressure may be increased to prevent the high-pressured gasket materialfrom flowing between electrolyte membrane 48 and gas diffusion layers50/52.

After gasket 46 is molded onto MEA 42, top block 12 and mold insert 16may open from bottom block 14, and the resulting gasketed MEA 42 may beremoved. Gasket 46 extends around the peripheral edges of MEA 42 toprevent pressurized gases and liquids from bypassing electrolytemembrane 48 during use in an electrochemical device (e.g., a fuel cell).Controlling the pressure applied to MEA 42 during the gasket moldingoperation allows gasket 46 to be securely connected to the peripheraledges of MEA 42 while also reducing the risk of over-compressing andunder-compressing MEA 42.

FIG. 4 is a cutaway perspective view of MEA 42 a and gasket 46, whereMEA 42 a is an alternative design of MEA 42, and includes electrolytemembrane 48 a and gas diffusion layers 50 a and 52 a. As shown,electrolyte membrane 48 a has a sawtooth-edge profile that extendsaround the entire peripheral edge of electrolyte membrane 48 a, withingap 53. The sawttooth-edge profile increases the surface area betweenMEA 42 a and gasket 46, thereby further strengthening the connectionbetween MEA 42 a and gasket 46. When injected into cavity 44, the gasketmaterial penetrates into gap 53, and conforms to the sawtooth-edgeprofile of electrolyte membrane 48 a. As discussed above, the pressurethat mold insert 16 applies to MEA 42 a may be adjusted to account forviscosity variations of the injected gasket material. This improves theextent to which the gasket material may penetrate into gap 53 andconform to the sawtooth-edge profile of electrolyte membrane 48 a.

FIGS. 5 a-5 c are top view illustrations of electrolyte membranes 48 a,48 b, and 48 c, respectively, which show examples of suitable edgeprofiles for MEA 42. In addition to the sawtooth-edge profile shown inFIGS. 4 and 5 a, suitable edge profiles for electrolyte membranes mayinclude T-indentations (electrolyte membrane 48 b) and slot indentations(electrolyte membrane 48 c). Essentially, any edge profile design thatincreases the contact area between MEA 42 and gasket 46 within gap 53may be used.

FIG. 6 is an expanded sectional view of a portion of molding system 110,which is a system similar to molding system 10 discussed above(corresponding reference labels are increased by “100”). Molding system110 is suitable for molding gaskets to MEA 142, which is similar to MEA42, except that electrolyte membrane 148 extends into cavity 144.Electrolyte membrane 148 may include subgasket layers (not shown)disposed on each side of electrolyte membrane 148 to further seal MEA142 and to provide mechanical support to the portion of electrolytemembrane 148 disposed within cavity 144.

As further shown, molding system 110 includes injection gates 140 a and140 b, where injection gate 140 a is identical to injection gates 40,discussed above in FIG. 2. Injection gate 140 b is also similar toinjection gates 40, except that injection gate 140 b extends throughbottom block 114. The use of injection gates 140 a and 140 b allowsgasket material to be injected into cavity 144 from each side ofelectrolyte membrane 148, thereby forming gaskets (not shown) on eachside of electrolyte membrane 148.

FIG. 7 is an expanded view of section 7 taken in FIG. 6, which showsgaskets 146 a and 146 b formed within cavity 144 adjacent the opposingsurfaces of electrolyte membrane 148. Upon solidification, gaskets 146 aand 146 b are secured to electrolyte membrane 148, and function in thesame manner as gasket 46 for preventing pressurized gases and liquidsfrom bypassing electrolyte membrane 148 during use.

FIG. 8 is a cutaway perspective view of MEA 142 a, which is analternative design of MEA 142, and includes electrolyte membrane 148 aand gas diffusion layers 150 a and 152 a. As shown, gas diffusion layers150 a and 152 a each have a sawtooth-edge profile that extends aroundthe entire peripheral edge of MEA 142 a, which improves the connectionbetween MEA 142 a and gasket 146. When injected into cavity 144, thegasket material conforms to the sawtooth-edge profiles of gas diffusionlayers 150 a and 152 a. The pressure that mold insert 116 applies to MEA142 a may also be adjusted to account for viscosity variations in theinjected gasket materials in the same manner as discussed above. Thisimproves the extent to which the gasket materials may conform to thesawtooth-edge profiles of gas diffusion layers 150 a and 152 a. Inalternative embodiments, gas diffusion layers 150 a and 152 a mayincorporate alternative edge profile designs, such as those discussedabove for electrolyte membranes 48 a-48 c in FIGS. 5 a-5 c.

FIGS. 9 a and 9 b are expanded sectional views of a portion of moldingsystem 210, which is an alternative system to molding systems 10 and 110discussed above (corresponding reference labels are increased by “200”from molding system 10). As discussed below, molding system 210 is atwo-shot molding system, which injects gasket material on each side ofelectrolyte membrane 248 of MEA 242.

As shown in FIG. 9 a, top block 212 and bottom block 214 do not includepatterned surface similar to patterned surfaces 28 and 36 of moldingsystem 10. Instead, molding system 210 includes cavity inserts 254 and256 located on opposing sides of electrolyte membrane 248. Cavityinserts 254 and 256 include patterned surfaces 228 and 236, whichfunction in the same manner as discussed above for patterned surfaces 28and 36 in FIG. 1.

Cavity insert 254 is secured to top block 212 such that patternedsurface 228 faces electrolyte membrane 248 to generally define topcavity 244 a. Cavity insert 256, however, is movably retained at anoffset location within cavity 244 from the surface of bottom block 214(referred to as surface 258). Cavity insert 256 is offset from surface258 at a distance that allows patterned surface 236 of cavity insert 256to support electrolyte membrane 248 during a first injection shot of thetwo-shot molding process.

While cavity insert 256 supports electrolyte membrane 248, gasketmaterial may be injected into top cavity 244 a from injection gate 240a. This substantially fills top cavity 244 a and allows the gasketmaterial to conform to electrolyte membrane 248, gas diffusion layer 250and patterned surface 228. Upon solidification, the gasket materialforms a first gasket (not shown), which is secured to electrolytemembrane 248 and gas diffusion layer 250. After the first gasketsolidifies within top cavity 244 a, cavity insert 256 may be loweredtoward surface 258 for injecting gasket material from injection gate 240b. At this point, the first gasket may function as a support forelectrolyte membrane 248 while gasket material is injected frominjection gate 240 b.

As shown in FIG. 9 b, when cavity insert 256 is disposed against surface258, patterned surface 236 and electrolyte membrane 248 generally definebottom cavity 244 b. Gasket material may then be injected into bottomcavity 244 b from injection gate 240 b. This substantially fills bottomcavity 244 b and allows the gasket material to conform to electrolytemembrane 248, gas diffusion layer 252, and patterned surface 236. Uponsolidification, the gasket material forms a second gasket (not shown),which is secured to electrolyte membrane 248 and gas diffusion layer252.

Cavity inserts 254 and 256 are suitable inserts for structurallysupporting electrolyte membrane 248 during multiple-shot gasket moldingoperations. This reduces the risk of damaging electrolyte membrane 248while the individual injections of gasket materials take place.

As discussed above, MEAs may be placed on the mounts of the moldingsystems 10, 110, and 210 in an automated or manual manner. In oneembodiment, the molding system of the present invention may be used in acontinuous process, where the MEAs (e.g., MEAs 42, 142, and 242) are fedto the molding system on a carrier belt (not shown) in an automatedmanner. In this embodiment, the carrier belt may pass directly over themount (e.g., mounts 18, 118, and 218) for positioning a given MEA on themount. The top block and the mold insert may then close against thebottom block to perform a gasket molding operation. When completed, thetop block and the mold insert may then open from the bottom block. Thecarrier belt may then remove the gasketed MEA and position a new MEA onmount 18. The process may then be repeated. The molding systems of thepresent invention are particularly suitable for use with continuousprocesses because the mold inserts (e.g., mold inserts 16, 116, and 216)may apply a constant pressure to each MEA, despite variations in layerthicknesses between the given MEAs. This reduces the time and effortrequired to manufacture MEAs.

Although the present invention has been described with reference topreferred embodiments, workers skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention.

1. A method for molding a gasket to a membrane electrode assembly havinga perimeter edge, the method comprising: positioning the membraneelectrode assembly between a first mold block a second mold block;closing the first mold block against the second mold block to define acavity, wherein the perimeter edge of the membrane electrode assembly isdisposed within the cavity; moving a mold insert to apply pressurearound the lateral edges of the membrane electrode assembly; andinjecting gasket material into the mold to form the gasket on theperimeter edge of the membrane electrode assembly.
 2. The method ofclaim 1, wherein the mold insert is moved until a preset pressure isapplied to the membrane electrode assembly.
 3. The method of claim 2,wherein the preset pressure is based at least in part on one or moreflow characteristics of the gasket material.
 4. The method of claim 1,further comprising retaining the membrane electrode assembly on a mount,wherein the cavity is disposed laterally around the mount.
 5. The methodof claim 1, further comprising forming a plurality of replicatedstructures in the gasket.
 6. The method of claim 1, wherein theperimeter edge of the membrane electrode assembly has a plurality ofedge shapes, and wherein the injected gasket material conforms to theedge shapes.